3,500m altitude: $K_alt \approx 0.92$
Derate by 1% to 2% per 300 meters above 2,000m.
Note: Altitude affects air cooling but NOT buried cables. The Physics: Non-linear loads (VFDs, computers, rectifiers) generate harmonic currents. Triplen harmonics (3rd, 9th, 15th) add in the neutral conductor, causing overheating even if phase currents are balanced. cable rating and derating factor
The NEC uses a formula based on conductor temperature rating.
At the heart of cable selection lies two critical concepts: (Ampacity) and Derating Factors . 3,500m altitude: $K_alt \approx 0
Trying a 240mm² cable (base rating ~ 450A): $450A \times 0.326 = 146.7A$ (Still slightly low).
| Altitude (m) | Approx. Factor | | :--- | :--- | | 0 – 2,000 | 1.00 | | 2,000 – 3,000 | 0.96 | | 3,000 – 4,000 | 0.92 | | > 4,000 | Engineering judgment | Triplen harmonics (3rd, 9th, 15th) add in the
Trying a 300mm² cable (base rating ~ 520A): $520A \times 0.326 = 169.5A$ (Sufficient for 150A load).